1. Technical Field
The present invention relates to an optical device used as; for instance, a projector light source.
2. Description of the Related Art
An optical device is proposed as a light source used; for instance, in a projection display device like a liquid crystal projector, assembled by a combination of a discharge lamp with; e.g., an elliptical surface reflecting mirror. The optical device is configured in such a way that light emitted from the discharge lamp undergoes reflection on the elliptical surface reflecting mirror, to thus enter an arbitrary optical system; for instance, a rod integrator or an integrator lens (a fly-eye lens), and irridiate an irradiation surface thereof. There recently exists an increasing demand for a brighter projection screen of a liquid crystal projector.
As shown in FIG. 7, an elliptical surface reflecting mirror 40 has a function of condensing light emitted from a first focal point F1 into a second focal point F2. However, in the optical device using such an elliptical surface reflecting mirror 40, when light rays that have been emitted at equidensity from a discharge lamp situated at the first focal point F1 on the elliptical surface reflecting mirror 40 is condensed to the second focal point F2, a ray density tends to become smaller with an increasing distance from an optical axis X of the elliptical surface reflecting mirror 40. There is also a problem of an area where light rays do not travel because of shading of the discharge lamp (an outlined area) existing in the vicinity of the optical axis X.
As shown in FIG. 8, a comparative technique is proposed to address such a problems (see Patent Document 1 (JP-A-2002-298625)). Specifically, an aspherical lens 45 is disposed, in its light exit direction, ahead of a reflector 40A. A shape of a reflecting surface 41 of the reflector 40A is adjusted in accordance with a shape of a light incident surface 46 or a light exit surface 47 of the aspherical lens 45, thereby adjusting an outgoing light distribution appearing on the light exit surface 47 of the aspherical lens 45 in such a way that the ray density comes to an equidensity. It makes the outlined area caused by the shading of the discharge lamp smaller.
More specifically, the reflecting surface 41 of the reflector 40A is given a shape that makes smaller a ray density of light rays incident on the aspherical lens 45 near a light axis X of the reflector 40A. Further, angles of the light rays exiting from the aspherical lens 45 are adjusted by the aspherical lens 45, thereby making uniform the ray density achieved on the light exit surface 47 of the aspherical lens 45. Namely, an angular interval dφ between the light rays on the light exit surface 47 of the aspherical lens 45.
However, when the shape of the reflecting surface 41 of the reflector 40A is designed so as to achieve, on the light exit surface 47 of the aspherical lens 45, an outgoing light distribution that makes ray density into equidensity, the size of the arc of the discharge lamp viewed from points of reflection on the reflecting surface 41 of the reflector 40A is not taken into account. It turned out that, for this reason, arc images appearing at a condensing position Q fail to assume constant size, which sometimes leads to a decrease in use efficiency of light and gives rise to a problem of generation of insufficient illuminance. Specifically, as shown in FIG. 9, when light rays emitted from tip ends 42a and 42b of respective electrodes enter an arbitrary reflecting position R5 on the reflecting surface 41, the light rays reflected at the reflecting position R5 enter the aspherical lens 45 while an angle α between the light rays achieved when the light rays enter the reflecting position R5 is maintained. Subsequently, an image based on the light rays from tip ends 42a and 42b is formed in size A at the condensing position Q. In the meantime, the light rays emitted from the tip ends 42a and 42b of the respective electrodes enter an arbitrary reflecting position R6 on the reflecting surface 41, the light rays reflected at the reflecting position R6 enters the aspherical lens 45 while an angle β (>α) between the light rays achieved when the light rays enter the reflecting position R6 is maintained. Subsequently, an image is formed at the condensing position Q. When ray density achieved on the light exit surface 47 of the aspherical lens 45 is made into equidensity, the image will be formed in a different size B (>A) at the condensing position Q. As a consequence, some of the light rays originating from the reflecting position R6 often fail to enter an aperture 50, which in turn generates unavailable light rays.